1. Field of the Invention
The present invention generally relates to fabrication of semiconductor devices and more particularly to a charged particle beam exposure system for exposing patterns such as a pattern of an integrated circuit, on an object such as a semiconductor substrate. More particularly, the present invention relates to a charged particle beam exposure system wherein a charged particle beam is deflected to pass through a selected shaping aperture provided on a stencil mask to form a shaped beam, and wherein such a shaped beam is used for exposing a pattern corresponding to the pattern of the selected aperture, on a substrate.
2. Description of the Related Art
An exposure system using a charged-particle beam, particularly the system using an electron beam, has been used for fabricating a high precision mask or a reticle, or for patterning an electron beam provided on a semiconductor substrate. Generally, a conventional electron beam exposure system utilizes a shaped electron beam having a rectangular cross section for the exposure, wherein such a shaped electron beam is produced by causing the electron beam to pass through a pair of rectangular shaping apertures successively. Such a pair of rectangular shaping apertures are disposed to overlap with each other partially when viewed in the traveling direction of the electron beam, and the size of the rectangular beam is adjusted by changing the overlapping relationship of the apertures. The rectangular-shaped electron beam thus produced is deflected over the substrate by a deflector under a control of a pattern generation unit. As a result, a desired pattern is drawn on the substrate as a succession of the rectangular patterns. Thus, the electron beam exposure system has established an important position in the fabrication technology of semiconductor devices due to its extremely high precision of exposure. By using the electron beam exposure process, one can fabricate submicron semiconductor devices without difficulty.
When the pattern to be exposed on the substrate becomes more intricate and minute, on the other hand, the number of the exposure shots increases sharply, and such an increase in the number of the exposure shots inevitably causes a problem in that the throughput of the exposure process is reduced. It will be easily understood that an exposure process for exposing a complicated semiconductor pattern on a substrate by moving a single focused electron beam, requires a substantial time. In order to improve the throughput of exposure, particularly the exposure of semiconductor devices that have superfine, submicron patterns, a block exposure process has been proposed. For example, the U.S. Pat. Nos. 5,051,556 and 5,173,582 describe such a block exposure process. Thus, the foregoing references are incorporated herein as reference. Generally, a typical LSI pattern includes a repetition of basic, or fundamental patterns. Thus, by shaping the electron beam according to one of such basic patterns of which number is limited and by repeating the exposure or shot of such a shaped electron beam a number of times, one can improve the throughput of the exposure significantly.
In order to achieve such a block exposure process, the electron beam exposure system mentioned above uses a stencil mask that carries thereon a plurality of apertures in correspondence to the basic patterns of the integrated circuit, and the stencil mask is disposed to interrupt the charged particle beam that travels form a beam source to the substrate along an optical axis. When an electron beam hits a selected aperture while traveling toward the substrate, the electron beam is shaped according to the shape of the selected aperture, and such a shape of the aperture is projected upon the substrate with a demagnification. Thereby, an exposure of the fundamental device pattern is achieved by a single shot of the electron beam thus shaped. By repeating the exposure shots a number of times while moving the electron beam over the substrate, the necessary pattern of the integrated circuit is exposed with a substantially reduced time.
In such a construction of the electron beam exposure system, the stencil mask carries thereon a large number of apertures, and the electron beam is deflected by a deflector to hit the selected aperture. A typical aperture may have a size of 500 .mu.m square on the stencil mask, and about one hundred such apertures are formed on the stencil mask. Thereby, it is necessary to deflect the electron beam over an area of as much as 6 mm square on the stencil mask by means of the deflector.
When implementing such a block exposure process, it is preferred that the electron beam exposure system satisfies the following conditions. First, the electron beam, deflected to hit the specific aperture on the stencil mask as described above, should impinge substantially perpendicularly upon the stencil mask. Second, the electron beam thus impinged upon the stencil mask has to create a sharp focused image thereon. Third, the electron beam deflected away from a central optical axis as a result of the deflection, should be deflected back to the optical axis after passage through the stencil mask. Fourth, a sharp focused image of the shaping aperture should be formed on the substrate.
In order to satisfy the foregoing requirements, particularly with respect to the first and third requirements, the foregoing U.S. Patents propose an electron beam exposure system wherein four deflectors are disposed along the optical axis of the electron beam for causing the foregoing addressing of the aperture of the stencil mask by the electron beam. More specifically, the first two deflectors are located at the upstream side of the stencil mask and the remaining two deflectors are located at the downstream side of the stencil mask, wherein the deflectors may either be an electrostatic deflector or an electromagnetic deflector. The first deflector deflects the electron beam away from the optical axis and the second deflector deflects the electron beam again such that the electron beam travels parallel to the optical axis. Thereby, the electron beam thus deflected by the second deflector impinges upon the stencil mask perpendicularly and is shaped according to the shape of the selected aperture on the stencil mask. The electron beam thus passed through the stencil mask is then deflected by the third deflector toward the optical axis, and the fourth deflector deflects the electron beam further such that the electron beam travels substantially coincident to the optical axis.
In such a block exposure system, however, due to the large beam displacement from the optical axis, there is a tendency that the electron beam exhibits aberration such as astigmatism or field curvature effect. When astigmatism occurs, the cross section of the shaped electron beam is modified and the image of the selected aperture is distorted on the substrate accordingly. When field curvature effect occurs, on the other hand, the focusing of the electron beam is modified and the exposed image tends to be blurred. Therefore, there has been a demand for a compensation mechanism that compensates for such a modification of the electron beam caused as a result of the electron beam traveling offset from the optical axis.
In the electron beam exposure system that uses the stencil mask, it should also be noted that the deflectors disposed at the upstream side and the downstream side of the stencil mask are driven simultaneously such that the electron beam hits the selected aperture on the stencil mask and returns to the optical axis again after passing through the mask. Thereby, it will be noted that the driving of the individual deflectors that causes such a deflection cannot be independent from each other. When the energization of one of the deflector is determined such that the electron beam hits a selected aperture, the energization of the rest of the deflectors is determined uniquely. This means that, in order to achieve the desired operation of the electron beam exposure system mentioned above, one has to obtain a function describing the relative driving energy of the four deflectors that are disposed above and below the stencil mask. However, the process for determining such a function includes an extremely complicated calibration process of the deflectors that includes a step of driving the four deflectors simultaneously while changing the combination of the driving energy variously for seeking an arrival of the electron beam on the substrate with a maximum intensity. Because of the fact that such a calibration has to be conducted periodically or whenever a stencil mask is changed, there has been a problem in the conventional electron beam exposure system in that the block exposure process requires a long calibration time and hence provides a low throughput, in spite of the increased efficiency of exposure associated with the efficient beam shaping. Further, the foregoing mechanism for compensating for the astigmatism or the field curvature effect has to be calibrated as a function of the energization of the deflectors, as such a modification of the electron beam occurs as a result of the offset of the electron beam from the optical axis and hence as a result of the energization of the deflectors.